Thank you @NedMan!
Even half a millimeter is virtually opaque to both the primary 10.4µm and secondary 9.4µm bands. But if those two sources are making equivalent measurements, acrylic has lower transmittivity in the near IR bands which could be generated from the lased material burning, and so is probably better eye protection than PC.
For the laser itself, it looks like I’d be safe with clear acrylic, and probably clear PC.
However, under what conditions does the lased material generate plasma? If it does, I would expect it to also emit broad-spectrum UV.
general purpose acrylic naturally filters all UV light below 345nm (100% UVB), but only 35% of UVA light.
OP-3 is a grade of acrylic that filters 98% of UVA as well. It is also possible to get PC with UV-blocking coatings applied. It’s also possible to get PET film dyed to absorb UVA, UVB, and UVC:
@Jammy I wonder whether this is the science behind using Plexiglass 2422, which absorbs light at wavelengths shorter than 540nm, for CO2 laser windows?